Beware of hidden price tags (not just in romance or with the opposite gender, btw). And just because he (or she) pays doesn't mean you therefore "owe" them (personally I never let a guy I didn't know well pay for me to prevent "misunderstandings"). Just because your existence is acknowledged with a smile or comes close doesn't mean the person who noticed you is coming onto you or even interested.
Sometimes people are incompatible and/or the chemistry isn't there. That isn't anyone's fault and neither side should feel bad over it, and it doesn't mean everyone else in the entire world is going to feel the same way. It doesn't mean you did anything wrong, nor do you have to invent reasons to "justify" why you're not feeling it either. And what one man or woman wants isn't what ALL (or even most) men or women want.
And speaking of chemistry, sometimes it's there but fades after a few months. I understand that happens a lot (familiarity breeds contempt?), and after it fades then all those "cute" aspects become annoying and downright intolerable.
If the person you're with is constantly talking trash or sharing secrets of others with you then they're talking trash about you and sharing your secrets behind your back. If they treat others like dirt then it won't be long before you're treated like dirt. If the person complains that everyone has screwed him or her over then it won't be long before that person says YOU screwed him/her over. STAY FAR AWAY!
Don't go to bars looking for romance, it's not worth it.
Some guys can feel "disrespected" if you talk to other guys while with them on a date (I think they believe he's coming onto you and you're OK with that). Likewise, a woman who is with a guy (or wants to be) who feels threatened by you can get vicious, even violent (especially if drugs like alcohol are involved), and can even blame you if he comes onto you when you clearly do NOT want his attention or harassment. It's just something to look out for...especially at bars.
Figure out what you want in a partner and go after it. Don't just sit pretty hoping your fantasy sweeps you off your feet as if you're guy then you'll almost certainly be ignored and if you're a woman you'll almost certainly be played. And be very, very careful that you're not seeing something that isn't there just because you WANT it to be there (OTOH some people always see what they fear instead).
Have good friends. Some friends will back stab you and try to mess you up. Choose better for yourself than that (otherwise why won't you settle for that kind of crap from a romantic partner, too?). The better kind will provided much needed reality checks (and might even save your life).
Anyway, that's enough to start with. I don't know enough about you to answer more specifically so I hope at least one thing there might prove useful. (And if I were to literally give advice to myself from the past it would be much more specific with names
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